Typically, lightweight foamed concrete is produced by introducing bubbles into lightweight concrete, and is configured such that a cured paste or mortar contains 20% or more of pores to thus decrease the density of concrete. When lightweight foamed concrete is produced only using a cement paste, it has a density of about 0.3 to 0.5 t/m2, and when mortar containing fine aggregate is used, the resulting concrete density falls in the range of 0.8 to 1.5 t/m2, and thus this concrete is mainly utilized in adiabatic applications in the form of blocks, panels and fillers on floors, walls, roofs and the like.
Lightweight foamed concrete is classified as either pre-foamed cellular concrete or post-foamed cellular concrete, depending on the sequence in which bubbles are generated. Pre-foamed cellular concrete, which is obtained by first generating bubbles and feeding the generated bubbles into a subsequent concrete manufacturing process, is employed with the goal of increasing lightweightness because the density thereof is lowered due to the pores therein, or of improving filling properties by increasing the slump using the ball-bearing effect of the bubbles.
With regard to pre-foamed cellular concrete, bubbles, which are preliminarily made, may be added to a cement paste (cement+water), mortar (cement+water+fine aggregate), or concrete (cement+water+fine aggregate+coarse aggregate), and are typically applied to a cement paste or mortar in order to realize desired lightweightness and filling properties. The use thereof in concrete that contains coarse aggregate is very rare. In particular, pre-foamed cellular concrete is formed by mixing preliminarily made bubbles with a cement paste or mortar so as to increase lightweightness and filling properties.
Also, lightweight foamed soil is used in the form of flowing concrete by blending pre-foamed bubbles with a cement paste and then mixing the blend with soil that has been sorted on-site in order to thus increase workability, and is a lightweight foamed material that has very low strength, thereby facilitating excavation for future repair. In this way, pre-foamed bubbles are utilized in non-structural concrete to ensure lightweightness and filling properties.
Pre-foamed cellular concrete is seldom used as a structural material. This is because an increase in air content by 1% results in a decrease in compressive strength of about 4%. For this reason, high-performance concrete, which uses coarse aggregate and is required to exhibit high strength, does not contain more than 20% air.
Compared to normal concrete comprising water, cement and aggregate, high-performance concrete exhibits high strength, high durability, and high flowability. Here, “high strength” means a strength of 35 MPa or more, “high durability” indicates a freeze-thaw resistance of 90% or more, and “high flowability” means concrete having very high workability. The above properties of high-performance concrete are obtained by using a powdered admixture, such as silica fume, flyash, slag powder, or metakaolin, and a chemical agent, such as an air-entraining agent, a high-performance flowing agent, or latex. Since both high strength and high durability have to be satisfied, the internal structure of the concrete is made dense, and the air content is controlled to about 3 to 6% to ensure freeze-thaw resistance.
Among the above materials, silica fume and a high-performance flowing agent are typically used in high-performance concrete. The silica fume is so fine that the size thereof is about 1/80 to 1/100 of the size of the cement powder for normal concrete. When silica fume is added to concrete, the inside of the concrete becomes denser due to the pozzolanic reaction that takes place therein, thereby assuring high strength and high durability. However, the specific surface area thereof is high, undesirably resulting in poor workability. Hence, silica fume is used together with a high-performance flowing agent.
Such silica fume is used in an amount of 7 to 10% based on the weight of the cement for normal concrete, and may be pre-blended with cement so as to be used in mixed cement form. When silica fume, which is very fine, is blended with other concrete materials upon blending concrete in an on-site batching plant (BP), silica fume is not uniformly dispersed, undesirably causing many problems. Furthermore, when silica fume is added in a powder phase to pre-blended concrete, it is not readily dispersed.
In order to produce high-performance concrete, a powdered admixture such as silica fume, flyash, blast slag powder, or metakaolin may be used. For dispersion of the admixture, mixed cement is utilized. In the case where the amount of mixed cement is large, an additional silo has to be provided in order to produce concrete in bulk in plants and then transfer it to the site. When the amount thereof is medium or small, concrete is supplied in the form of ton bags, undesirably increasing production costs due to additional processing and transport.
Moreover, when high-performance concrete is produced and supplied in an on-site batching plant, the entire cross-section of the structure of interest has to be constructed from high-performance concrete, undesirably increasing construction costs.